The Trans-Pacific Partnership, one of the world’s biggest multinational trade deals, was signed by 12 member nations on Thursday in New Zealand, but the massive trade pact will still require years of tough negotiations before it becomes a reality.

The TPP, a deal which will cover 40 percent of the world economy, has already taken five years of negotiations to reach Thursday’s signing stage.

The signing is “an important step” but the agreement “is still just a piece of paper, or rather over 16,000 pieces of paper until it actually comes into force,” said New Zealand Prime Minister John Key at the ceremony in Auckland.

The TPP will now undergo a two-year ratification period in which at least six countries – that account for 85 percent of the combined gross domestic production of the 12 TPP nations – must approve the final text for the deal to be implemented.

The 12 nations include Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore, the United States and Vietnam.

Given their size, both the United States and Japan would need to ratify the deal, which will set common standards on issues ranging from workers’ rights to intellectual property protection in 12 Pacific nations.

Opposition from many U.S. Democrats and some Republicans could mean a vote on the TPP is unlikely before President Barack Obama, a supporter of the TPP, leaves office early in 2017.

U.S. Trade Representative Michael Froman has said the current administration is doing everything in its power to move the deal and on Thursday told reporters he was confident the deal would get the necessary support in Congress.

In Japan, the resignation of Economics Minister Akira Amari – Japan’s main TPP negotiator – may make it more difficult to sell the deal in Japan.

There is wide spread grassroots opposition to the TPP in many countries. Opponents have criticized the secrecy surrounding TPP talks, raised concerns about reduced access to things like affordable medicines, and a clause which allows foreign investors the right to sue if they feel their profits have been impacted by a law or policy in the host country.

In New Zealand on Thursday more than 1,000 protesters caused traffic disruptions in and around Auckland and police said a large number of police have been deployed.

Australian Trade Minister Andrew Robb said the agreement would be tabled next week in parliament. Opposition to the deal in Australia has been building, but Robb was confident it would be approved, despite the government not control the Senate.

Canada’s new government signed the deal on Thursday, but Trade Minister Chrystia Freeland has said “signing does not equal ratifying.”

She emphasized that the government committed itself to a wide-ranging consultation on the TPP during its election campaign and that process was currently underway.

Secretary of the Economy for Mexico, Illdefonso Guajardo, said the TPP would be voted on before the end of 2016, while Malaysia said the deal had already been approved, although some legislative changes were still needed.

Statement by U.S. President Barack Obama on the Signing of the Trans-Pacific Partnership

For more than five years, a group of 12 countries have negotiated a forward-looking trade deal that sets new, high standards for trade and investment in one of the world’s fastest growing and most important regions. Today, these countries signed the Trans-Pacific Partnership – a new type of trade deal that puts American workers first.

Right now, the rules of global trade too often undermine our values and put our workers and businesses at a disadvantage. TPP will change that. It eliminates more than 18,000 taxes that various countries put on Made in America products. It promotes a free and open Internet and prevents unfair laws that restrict the free flow of data and information. It includes the strongest labor standards and environmental commitments in history – and, unlike in past agreements, these standards are fully enforceable. TPP allows America – and not countries like China – to write the rules of the road in the 21st century, which is especially important in a region as dynamic as the Asia-Pacific.

Put simply, TPP will bolster our leadership abroad and support good jobs here at home. That’s why I released the full text of the agreement three months ago for all to see, and it’s why I’ll continue working with Democrats and Republicans in Congress to enact it into law as soon as possible so our economy can immediately start benefiting from the tens of billions of dollars in new export opportunities. We should get TPP done this year and give more American workers the shot at success they deserve and help more American businesses compete and win around the world.